This invention relates to energy conservation and more particularly to a damper type device for regulating the combustion air supplied to an oil or gas burning furnace in such a way that while the furnace is operating an optimum amount of combustion air is supplied to the combustion chamber thereof but while the furnace is temporarily shut down between periods of operation, the air supply to the combustion chamber is cut off to minimize heat loss up the flue.
After an oil burner shuts down, its heat exchanger is generally at maximum temperature and heat therefrom can be carried up the chimney and lost through convection currents. When the furnace later re-starts, this lost heat must be replaced resulting in longer periods of furnace operation to achieve a given amount of heat supplied to the area to be heated. As the heat is lost up the chimney, a partial vacuum is created inside the combustion chamber of the furnace. The resulting suction will bring in air along the path of least resistance from the area surrounding the furnace. This path is usually through the combustion air intake system. Most oil burning furnaces include an air blower or fan usually of the centrifugal type for supplying combustion air to the furnace. These blowers normally take the air from the room in which the furnace is located through an annular array of holes forming part of the blower frame. Often a shutter arrangement is provided to control and optimize the amount of air admitted via these holes. Thus, there is an air path through such shutter to the combustion chamber through which air will be sucked to replace the hot air lost up the chimney between periods of oil burner operation.
Several different means have been devised to reduce to eliminated this heat loss. The combustion air blowers have been provided with butterfly type valves at their inputs which are automatically opened by a hydraulic piston and cylinder in response to pressure of the fuel oil resulting from the switching on of the furnace. The butterfly valve is closed by the action of gravity when the oil pump and blower are de-energized, to prevent ambient air from entering the blower input. Such a system is shown in U.S. Pat. No. 2,702,589, issued Feb. 22, 1955. Air blower inputs have also been provided with electrically operated shutters; see U.S. Pat. No. 4,155,699 dated May 22, 1979. By restricting the amount of ambient air entering the combustion chamber through the air blower, the devices of these two patents cut off the air supply which makes the convective heat loss possible. Another approach to this problem has been the use of electrically operated dampers in the furnace flue to cut off the convection currents when the furnace is not operating. These devices pose some safety problems, since a malfunction of the damper which leaves it closed while the furnace is operating will result in backup of smoke into the basement and the house.
The present invention comprises a device which greatly reduces the heat loss described above by means of a simple, fail-safe device which requires no electricity for its operation and can be easily retrofitted to most existing oil burners. The novel damper comprises a valve which opens in response to the suction of the air blower to provide the furnace with combustion air, and closes due to gravity when this suction stops as the blower is de-energized.